Re-zeroing at altitude

[sinister]

In my experience with .308 Winchester and .338 Lapua Magnum your 100-yard zero will NOT change.

Where you are going to find variance is in the amount of drop you will have at your extended distances at altitude due to thinner air density.

You will gain about 25 to 45/50 feet per second in muzzle velocity per 1,000 feet of elevation -- the equivalent of another inch of barrel length. You won't need to dial in as much elevation correction the higher you go.

My experience is based on transitions between near sea level (San Joaquin Valley in California; Fort Lewis, Washington; Fort Bragg and Camp Butner, North Carolina) to ranges at altitude (4,500-5,000 feet at Dugway Proving Ground, Utah; Rexburg, Idaho; Rio Rancho, New Mexico; Bozeman, Montana; 6,000 feet at Park City, Utah, and 6,680 feet at Raton, New Mexico; and 10,000 feet in the Uinta Mountains in Utah).

 

[GreenDecoy]

You don't have to zero your rifle at 100 to know what your 100 yard zero is at a given altitude. Using 100 yards is convenient for the ballistic apps.

But say I zero at 300 yards at sea level. The zero may be off a little at 10k'. But you can adjust it using AB or any app based on altitude. You can also determine your hundred yard zero quite easily too.

I always sight in at 100 and then dial up dope for MPBR at altitude. My dope chart tells me how much I need to dial down to reset my 100 yd zero.

It looks like about a 3" difference for most cartridges at 500 yds going between sea level and 10000. Temperature has major impact too because air density is all that matters for drag. The density altitude on a hot muggy day at sea level could be over 3000'. So the difference between a cold mountain day at 10k' and the firing line outside Atlanta in July may be less than you might think.

 

[Joefrazell]

My zero hasn't changed from 2600' to 7000' when checking but I'm sure it could. Always check your zero

 

[waveslayer]

So I live at sea level and I always have to re-zero my rifles when I go hunting. Especially at altitude my 6.5 Creedmoor at 8,000' will shoot low by .4 mils. It will depend on your load you are shooting.

So what I do is the following: I input into my ballistics calculator an average temp and the average altitude that I will be hunting at. I see what the 100 yard zero will be according to my app. SO, if it says I need to adjust .4 mils, I re-zero it to .4 mils and verify it if I can. I have done this several times and I have been dead on each time. If I didn't re-zero I would have been shooting low or high, depending on what gun I am using and load data.

Get a good ballistics app like Applied Ballistics, Hornady 4DOF, and TRASOL.

 

[Idaho Hunter1]

I live in ID and regularly shoot at 2700'msl. I have dope cards for 5k and 8k msl that I hunt with. There is no essential difference inside 600yds regarding drop. What does come into play is wind holds and energy of the bullet will be higher. My limit for energy to shoot an elk is 1200ftlbs. This happens much further out at 8k msl than down in the Boise valley.

I've found AB to be accurate when set up for station pressure and altitude and to correct for a 2700' msl zero without issue.

Good luck